1. Technical Field
This invention relates to methods, instruments and apparatus for optically characterising eye-related optical systems, preferably over wide angles of view. Eye-related optical systems include the natural animal eye, alone or in association with prosthetic lenses and with or without surgical or other modification. They also include physical eye models or simulated eyes with or without modification to simulate optical disorders and/or corrective measures.
Optical characterisation typically involves refractometry; that is, the determination of the optical power of portion or the entire optical path traveled by an interrogating ray and it may, for example, include mapping—or spatially resolving—refractive power over an area or surface of the eye-related system, which is sometimes referred to as wavefront aberrometry. Optical characterization may also include determination of the length of the eye-related system, for example the distance from the anterior surface of the cornea to the anterior surface of the retina. Other characteristics of natural eyes, such as the profile and thickness of the cornea, pupil size and the depth of the anterior chamber, may also be important for certain surgical procedures (eg, lens replacement or ablative laser treatment) but are not of prime concern in this invention.
Of particular—but not exclusive—interest are methods and instruments suitable for use by optometrists in determining the peripheral refraction—and, optionally, the length—of the human eye for the purpose of prescribing anti-myopia treatment. Peripheral angles of 20-30 degrees with respect to the optic axis are of particular interest in this respect, with angles up to about 40 degrees also being relevant. Even higher peripheral angles are of research interest to specialists.
2. Description of Related Art
Several methods and instruments have been used to measure central refraction errors and aberrations of the eye. Refractive error is a subset of the total aberration of the eye and traditionally expressed in terms of sphere and cylinder components along with the orientation of the cylinder axis. Although it is possible to extract the refractive error, also called lower order aberrations, from the measurements of total aberration, the instruments used in clinical practice are usually dedicated to either measurement of refractive error or total aberration. While neither instrument is designed for measurement of peripheral refraction or aberration, commercial instruments have been modified to obtain measurements of peripheral refraction for both the accommodated and unaccommodated eye. The modifications include some form of off-centre fixation with and head or eye movement being needed.
Atchinson [Atchinson D A. “Comparison of peripheral refractions determined by different instruments”. Optom Vis Sci 2003; 80:655-60] describes one such comparison of two auto-refractometers (Canon Autoref R-1 and Shin-Nippon SRW-5000) and one Hartmann-Shack wavefront aberrometer. With all three instruments, peripheral refraction or total aberration was measured by rotating and fixating the eye on a series of fixation targets along the horizontal meridian of up to 40° nasal and temporal. All measurements were taken sequentially, with patients being instructed to fixate on a particular target and then re-centering the pupil position with the optical axis. Overall, the three instruments produced similar results, although the Canon results are more variable. Several similar investigations have been published using similar methods and instruments and obtaining similar results. [Gwiazda J, Weber C. Comparison of spherical equivalent refraction of astigmatism measured with three different model of autorefractors. Optom Vis Sci 2004; 81:56-61, and Gustafsson J, Terenius E, Buchheister J, Unsbo P. Peripheral astigmatism in emmetropic eyes. Ophthalmic Physiol Opt 2001; 21:393-400.]
A number of different optical methods have been utilised to automatically determine the refractive status of the live eye. The basic principle employed is the projection of an optical pattern or beam onto the retina and the analysis of the reflected pattern. An overview of these methods is given in [Atchinson D A. “Recent advances in measurement of monochomatic aberrations of human eyes”. Clin Exp Optom 2005; 88: 1: 5-27]. One of the most commonly used principles is used in the Shin-Nippon SRW-5000 instrument in which an infrared ring target is projected onto the retina and the reflected image is captured with a CCD camera. A lens relay system moves rapidly, scanning through the focus range and the size of the images are analysed in multiple meridians to provide the data from which the aberrations (including refraction) can be derived. Some of these techniques have the advantage of being ‘open-field’ in that the subject can look through a glass window and semi silvered mirror into the distance, thus preventing instrument myopia, but also allowing fixation at off axis angles. Typically, the angular fixation range is limited to less than 30° in the horizontal and about half of that in the vertical meridian. This technique is not sufficient to fully characterize peripheral aberration with and without vision correction devices.
A similar instrument was described and used in a laboratory setting by Artal et al. [Artal P, Derrington A M, Colombo E. “Refraction, aliasing, and the absence of motion reversals in peripheral vision”. Vision Res 1995; 35: 939-47.] A point image is projected onto the retina. The reflected image is observed with a CCD camera while moving the ‘focusing block’ axially until the best focus position with smallest circle of confusion was found. To assess astigmatism, the positions for sharpest horizontal and vertical profiles were also determined. A fixation target was placed at comfortable viewing distance in locations for 15°, 20° and 40° retinal eccentricities in the horizontal meridian.
Webb et al describe a modified Scheiner system whereby the subject manipulates the incidence angle of one of the Scheiner beams until two dots on the retina merge into one. [Webb R H, Penney C M, Thompson K R “Measurement of ocular local wavefront distortion with a spatially resolved refractometer”. Appl Opt 1992; 31: 3678-3686.] Although the measurement beam enters the pupil non-parallel to the optical axis, the angular deviations are very small and only compensate the paraxial wavefront error of the eye. No peripheral refraction measurements appear possible with this system.
In 2003, Schmid presented results of peripheral axial length measurements from an instrument developed utilising optical low coherence reflectometry. [Schmid G F. “Axial and peripheral eye length measured with optical low coherence reflectometry”. J. Biomed. Opt. 2003 8(4): 655-662. See also, Schmid et al, “Measurement of eye length and eye shape by optical low coherence reflectometry”. Intnl. Opth. 2001 23(4-6).] A fixation LED was coupled into the central optical path to keep the eye aligned with the optical axis of the instrument. A beam steering mirror deflects the measurement beam horizontally and vertically by up to 15° for off-axis measurements. The measurement principle requires the incident beam to be aligned perpendicular to the cornea at the point of intersection. Due to the non-spherical shape of the cornea, small lateral repositioning of the instrument is necessary for each new incident angle. This manual process prohibits rapid measurements across the angular range.
In U.S. Pat. No. 6,439,720, Graves et al disclose an instrument for measuring lower and higher order aberrations of the human eye. The method described is one of several variations of double pass techniques, whereby a probing light beam illuminates a spot on the retina and the reflected wavefront is analysed after exiting the eye. In this patent, a pair of Littrow prisms is used to split the emerging light ray into two parallel beams which pass through a moveable collimating lens to generate two slightly defocused images on a CCD detector. From the two computer analysed images, the ocular aberrations can be determined. The patent describes only on axis measurements of lower and higher order aberrations.
Wei et al [US patent 20052034] disclose a multifunctional instrument to measure axial eye length and corneal topography. Although not directly dedicated to obtain aberration and refraction results, the instrument features several sub-components also used in aberrometry and the combination of axial length parameters and keratometry data allows some estimation of the refractive status. Again, the instrument is designed for on axis measurements only. The fixation target can be moved but only along the optical axis to stimulate different accommodative responses.
An instrument to measure aberrations of the eye at a plurality of locations was disclosed by Molebny at al [U.S. Pat. No. 6,409,345]. The plurality of locations are achieved by parallel offsetting the probing light beam with respect to the optical axis. Aberration mapping is thus confined to paraxial scanning to obtain power maps across the entrance pupil. As with Wei et al (above), a fixation target is added to align the optical axis and to control accommodation.
The techniques outlined above are generally unsatisfactory because the patient is unable to correctly fixate gaze for the time needed and because the peripheral angle at which the measurement is taken is difficult to measure with accuracy or reliability. Attempting to map peripheral power even over a few spots on the eye in one sitting is impractical and repeatability between different sittings is generally poor.
Neal et al [U.S. Pat. No. 6,634,750] disclose a ‘tomographic wavefront analysis system and method of mapping an optical system’ in which interrogating beam(s) are scanned into multiple locations within the eye and back-scattered light from is detected and processed into an aberration map or representations of the three-dimensional structure of the eye using computer automated tomography. While the difficulty of peripheral gaze fixation is avoided, the system is highly complex, unsuited for use in normal optometry practise and appears to be incapable of interrogating an eye at peripheral angles greater than about 10-15 degrees with respect to the optic axis. Further, the disclosed technique is only suited to the use of spot beams and does not envisage or permit the use of interrogating beams having various cross-sectional shapes, such as squares, circles, ellipses or rings which assist in auto focussing/ranging and accelerate wavefront analysis.
Methods and instruments that are capable of more accurate peripheral refraction measurements over wide peripheral angles are needed to provide important inputs for the determination of ocular shape, eye length or retinal contour. Such inputs are now of significant interest in the study and treatment of eye pathologies such as progressive myopia.